This invention relates to the biological treatment of wastewater and provides a method and apparatus for both the rapid upgrading in efficiency of existing activated sludge wastewater treatment plants and for the design of new and more efficient activated sludge treatment plants. More specifically, the invention is concerned with utilizing a rotatable, partially submerged contacter in the aeration tank of an activated sludge wastewater treatment plant.
At the present time, wastewater treatment plants commonly utilize one or more processes for the treatment of wastewater. The most widely used watewater treatment process is that of "primary treatment" which relies on the plain sedimentation of settleable wastewater solids for biochemical oxygen demand BOD and suspended solids removal. The efficiency of the primary process is in the 25-30% range. However, the results of primary treatment can be improved by the use of chemicals to enhance the settleability of wastewater solids.
In addition to the primary treatment process, there are a number of "secondary treatment" processes employed. These processes improve water quality by employing the growth of biological masses which utilize the pollutants in the water for nutrients in their metabolic life cycles. The subsequent removal of the biological masses from the wastewater results in an effluent of an improved quality.
The "trickling filter" treatment is one secondary technique widely used in the treatment of wastewater. The filter is a packed bed of media which provides a surface upon which a film of biological slime can grow and over which the wastewater is passed. The oxygen and organic matter in the wastewater diffuses into the film where the oxidation and synthesis of additional growth can occur. Plants using the trickling filter method can effect a carbonaceous BOD removal of 60 to 90 percent, depending mainly on the loading rate of the facility. In addition, the trickling filter process can effect an oxidation of the ammonia which is present in the wastewater which is desirable.
The other and probably most widely used secondary treatment for wastewater is the "activated sludge" process. The activated sludge process can be defined as a process in which flocculated biological growths are continuously circulated and contacted with organic wastewater in the presence of oxygen. The oxygen is usually supplied by bubbling air into the sludge-liquid mixture in an aeration tank. This air can be introduced through air diffusers to develop a hydraulic motion of the contents of the aeration tank. A typical hydraulic motion involves a "spiral roll" of the contents of the aeration tank. The aeration step is usually followed by a solids-liquid separation, from which a portion of the biologically active sludge is separated and recycled back to the aeration tank to provide an active source of bacterial growth to continue treatment.
The activated sludge process under optimum conditions can be up to 90% effective in the removal of carbonaceous BOD; it can also be effective to oxidize the ammonia present. However, the "activated sludge" process is not without disadvantages as it can be readily upset by surges in the volume of wastewater and other circumstances which may prevent the attainment of the designed goals.
In recent years, another wastewater treatment technique has become popular which employes "rotating film" contactors also known as rotating biological contactors. Generally, the fixed film contactors are comprised of a plurality of plastic discs which are relatively densely packed but spaced apart to form cylinders. The surfaces of the discs provide the media upon which bacteria and other simple life forms can attach and grow. Unlike the filter beds of the trickling filter, the discs are only partially submerged in the wastewater and can be rotated to provide a continuous supply of nutrients and oxygen to the microorganisms. The shearing forces exerted on the organisms during rotation through the wastewater cause excessive growth to slough from the media into the mixed liquor. The rotation of the contactors provides a mixing action which keeps the biomass in suspension, and the wastewater flow carries the solids out of the system for subsequent clarification.
The rotating fixed film process differs from the trickling filter process in that it has a dynamic, rather than a stationary media and the media is exposed to more wastewater. And, it differs from the activated sludge process by having an attached, rather than a suspended biomass. In addition, the rotating fixed film process achieves higher levels of treatment than those obtained with the trickling filter method. It also possesses an advantage over the activated sludge process in that it has less susceptibility to upset from the changes in hydraulic or organic loadings.
Among the patents that have issued relating to the rotating fixed film process and the Torpey U.S. Pat. No. 3,575,849 which discloses the use of forcibly rotated contactors in secondary sedimentation tanks, but also mentions that forcibly rotated contactors could be used in the aeration tank of an activated sludge wastewater treatment plant, and the Prosser U.S. Pat. No. 3,886,074 which discloses the use of air driven rotating contactors.